Polysilicon surface micromachining adapts planar fabrication process steps known to the integrated circuit (IC) industry to manufacture microelectromechanical or micromechanical devices. The standard building-block processes for polysilicon surface micromachining are deposition and photolithographically patterning of alternate layers of low-stress polycrystalline silicon (also termed polysilicon) and a sacrificial material (e.g. silicon dioxide or a silicate glass). Vias etched through the sacrificial layers at predetermined locations provide anchor points to a substrate and mechanical and electrical interconnections between the polysilicon layers. Functional elements of the device are built up layer by layer using a series of deposition and patterning process steps. After the device structure is completed, it can be released for movement by removing the sacrificial material using a selective etchant such as hydrofluoric acid (HF) which does not substantially attack the polysilicon layers.
The result is a construction system generally consisting of a first layer of polysilicon which provides electrical interconnections and/or a voltage reference plane, and up to three or more additional layers of mechanical polysilicon which can be used to form functional elements ranging from simple cantilevered beams to complex systems such as an electrostatic motor connected to a plurality of gears. Typical in-plane lateral dimensions of the functional elements can range from one micron to several hundred microns, while the layer thicknesses are typically about 1-2 microns. Because the entire process is based on standard IC fabrication technology, a large number of fully assembled devices can be batch-fabricated on a silicon substrate without any need for piece-part assembly.
A microelectromechanical (MEM) engine or micromotor comprising a pair of linear actuators driven 90.degree. out-of-phase to rotate an output drive gear is disclosed in U.S. Pat. No. 5,631,514 to Garcia et al, which is incorporated herein by reference. This micromotor can be used for actuation of complex MEM mechanisms and devices via a multi-gear transmission. However, some types of MEM devices require a precisely indexed motion over a small angle of rotation or over a small range of linear translation. Such precisely indexed motion is problematic when using the micromotor of Garcia et al due to an imprecise motion of the output drive gear. What is needed is an alternative actuator mechanism that can be used to provide a precise and controlled motion of a moveable member such as a rotary gear or a rack.
An advantage of the present invention is that an indexing apparatus is provided which can be used to smoothly and precisely rotate a gear over a predetermined angle, or to translate a rack over a predetermined distance.
Another advantage of the present invention is the amount of angular rotation or linear translation of a moveable member can be determined from an electrical drive signal supplied to the indexing apparatus.
Yet another advantage of the present invention is that the indexing apparatus can be operated with different types of actuators including electrostatic actuators, thermal actuators and electromagnetic actuators.
Still another advantage of the present invention is that the indexing apparatus can be used to control the motion of moveable members generally having lateral dimensions in the range of a few microns to a few millimeters.
These and other advantages of the method of the present invention will become evident to those skilled in the art.